The invention relates to a ballast circuit for operating a discharge lamp comprising
input terminals for connection to a supply voltage source,
a rectifier coupled to the input terminals for rectifying a low frequency supply voltage supplied by the supply voltage source,
a buffer capacitor circuit coupled to an output of the rectifier,
an inverter coupled to the buffer capacitor circuit for generating a high frequency lamp current out of a DC voltage that is present over the buffer capacitor circut during operation, said inverter comprising a control circuit for controlling the operation of the inverter.
Such a ballast circuit is known from WO 98/46054.The known ballast circuit is equipped with a dimming circuit for controlling the light output of the discharge lamp in dependency of a dimming signal and with a conversion circuit for converting the shape of the low frequency supply voltage present between the input terminals into a dimming signal. The shape of the low frequency supply voltage in turn depends on the phase angle of a TRIAC dimmer that is used in combination with the known ballast circuit. Thanks to the presence of the dimming circuit and the conversion circuit, the ballast circuit is TRIAC dimmable. When the phase angle of the TRIAC dimmer is changed, the shape of the low frequency supply voltage changes accordingly so that the dimming signal generated by the conversion circuit and therefore the light output of the discharge lamp change as well. Up to a phase angle of 90 degrees the DC voltage that is present over the buffer capacitor means remains substantially unchanged. For phase angles higher than 90 degrees, however, the DC voltage that is present over the buffer capacitor means decreases when the phase angle is increased. When the voltage over the buffer capacitor means drops too much the control circuit will receive insufficient energy to operate properly and as a result of that the discharge lamp will extinguish. After the discharge lamp has extinguished the power consumption of the ballast circuit is much lower so that the DC voltage over the buffer capacitor means will increase. After a certain amount of increase of the DC voltage the control circuit is activated again and will ignite the discharge lamp. Once the discharge lamp has ignited the power consumption of the ballast circuit is increased so much that the DC voltage over the buffer capacitor means drops once more resulting in an almost immediate extinguishing of the discharge lamp. The cycle of events described hereabove repeats itself time after time resulting in flickering of the discharge lamp. Besides the fact that this is unpleasant to look at, the electrodes will be damaged since the lamp ignites without the electrodes having been preheated in a proper way.
The invention aims to provide a ballast circuit that is TRIAC dimmable but does not have the disadvantages mentioned hereabove.
A ballast circuit as mentioned in the opening paragraph is therefor according to the invention characterized in that the ballast circuit comprises a shut off circuit for switching off the control circuit if the DC voltage over the buffer capacitor circuit (hereinafter referred to simply as buffer capacitor drops below a first predetermined value.
If a ballast circuit according to the invention is used in combination with a TRIAC dimmer and a user chooses a phase angle that is too high, the DC voltage over the buffer capacitor means drops below the first predetermined value and the shut off circuit switches the control circuit off so that the discharge lamp extinguishes and flickering of the discharge lamp is prevented.
It is for instance possible to implement the shut off circuit in such a way that it is equipped with a reactivation switch for the user of the ballast circuit. In that case the control circuit remains switched off until a user of the ballast circuit has used the reactivation switch. Such a reactivation switch could be implemented as the mains switch for electrically connecting the ballast circuit to the supply voltage source. However, unless the phase angle of the TRIAC dimmer has been adjusted to a lower value, after reactivation the control circuit will be switched off again virtually immediately after the discharge lamp has ignited. As a result, in case a user of the ballast circuit chooses a value of the phase angle that results in the switching off of the control circuit, that user will have to use both the TRIAC dimmer and the reactivation switch to reactivate the ballast circuit. Since this is a strong disadvantage the shut off circuit preferably comprises a hysteresis circuit for reactivating the control circuit after the first shut off of the control circuit if the DC voltage over the buffer capacitor means rises above a second predetermined value that is higher than the first predetermined value, and switching off the control circuit when the DC voltage over the buffer capacitor drops below the second predetermined value.
When the shut off circuit has switched off the control circuit and the discharge lamp has extinguished, the ballast circuit no longer consumes any power or consumes only a relatively small amount of power. This results in an increase of the DC voltage that is present over the buffer capacitor. Because the second predetermined value is higher than the first predetermined value this increase does not result in an immediate reactivation of the control circuit. The control circuit is only reactivated when the DC voltage over the buffer capacitor has increased substantially. It is to be noted that the second predetermined value must not be chosen too high, since that would prevent the reactivation circuit from reactivating the control circuit even if a user decreases the phase angle of the TRIAC dimmer to a value that allows stable lamp operation, after the control circuit has been shut off.
If the ballast circuit does not consume any power after the shut off circuit has switched off the control circuit, the TRIAC dimmer carries no load current after the shut off of the control circuit. In practice it has been found that often under these circumstances the TRIAC dimmer fires at random. As a consequence the voltage over the buffer capacitor means reaches its maximum value and the control circuit is reactivated. However, as soon as the discharge lamp has ignited, a load current is once more present so that the TRIAC dimmer fires at the adjusted phase angle and the voltage over the buffer capacitor means drop below the first predetermined value so that the control circuit is switched off and the discharge lamp extinguishes. After the extinguishing of the discharge lamp the TRIAC dimmer once more fires at random etc. so that flickering of the discharge lamp results. This flickering can be prevented by assuring that there is a load current present even when the control circuit has been switched off. This can be realized in an effective manner by connecting the input terminals by means of a circuit part that carries a current as long as the low frequency supply voltage has an amplitude that differs from zero. Preferably the circuit part comprises an ohmic resistor.
In case the control circuit is reactivated after having been shut off, it will generally first control the operation of the inverter in such a way that the electrodes of the discharge lamp are preheated. The power consumption of the ballast circuit is relatively low during preheating when compared with the power consumption during ignition and during normal or dimmed operation. In spite of the fact that the power consumption is relatively low, the second predetermined value is chosen so that this power consumption will cause the DC voltage over the buffer capacitor to drop below the second predetermined value while the ballast circuit is still preheating the electrodes of the discharge lamp. As a result the control circuit is once more switched off before the discharge lamp has ignited and therefor flickering is prevented. After the control circuit has been switched off, the ballast circuit no longer supplies a preheat current to the electrodes of the discharge lamp so that the DC voltage over the buffer capacitor once more increases to the second predetermined value so that the reactivation circuit reactivates the control circuit and the electrodes of the discharge lamp are heated until the control circuit is switched off again. In case, however, a user of the ballast circuit lowers the phase angle of the TRIAC dimmer to a sufficient extent, the DC voltage over the buffer capacitor will increase to a higher value and will not drop below the second predetermined value during the preheat of the electrodes. In that case the discharge lamp may be ignited once more and burn steadily after ignition. Since the power consumption of the ballast circuit will generally increase during ignition and normal or dimmed operation, it is desirable to deactivate the hysteresis circuit after the preheating of the electrodes. In other words, in case the control circuit successively controls the operation of the ballast circuit to preheat electrodes of the discharge lamp, to ignite the discharge lamp and to operate the discharge lamp, the shut off circuit preferably comprises a deactivation circuit for deactivating the hysteresis circuit after the preheating of the electrodes of the discharge lamp.
The continuous repetition of electrode heating that takes place in a ballast circuit according to the invention, in which the shut off circuit is equipped with a hysteresis circuit, causes such a high temperature of the electrodes that they are damaged as a result of this temperature. To prevent such damage the shut off circuit preferably comprises a delay circuit for delaying the reactivation of the control circuit after the DC voltage over the buffer capacitor means has reached the second predetermined value. Since the control circuit is only reactivated after a delay, the electrodes of the discharge lamp can cool down longer before another preheat cycle starts so that the average temperature of the electrodes is lower. The delay means can be implemented in a relatively simple and cheap way by making use of a resistor, a capacitor and a diode or a zener diode.
In practice the control circuit of an electronic ballast circuit often comprises an integrated circuit. In that case the electronic ballast further includes a supply circuit part coupled with the control circuit for generating a DC supply voltage for the control circuit. Such an integrated circuit is often equipped with a switch off circuit part for switching off the control circuit in case the DC supply voltage drops below a predetermined lock out voltage. In case such a switch off circuit is comprised in the control circuit, the shut off circuit can be realized in a relatively simple way in case the shut off circuit part comprises means for clamping the DC supply voltage to a fraction of the DC voltage that is present over the buffer capacitor means. A proper choice of the fraction will make sure that when the DC voltage over the buffer capacitor means drops below the first predetermined value, the DC supply voltage of the control circuit drops below the predetermined lock out voltage so that the switch off circuit switches the control circuit off. It has been found that this clamping could be realized in an efficient and simple way making use of a bipolar transistor.
In a ballast circuit in which the DC supply voltage is clamped to a fraction of the DC voltage over the buffer capacitor means, the hysteresis circuit can relatively easily be realized in case the shut off circuit comprises a fraction decrease circuit part for decreasing the fraction of the DC voltage after shut off of the control circuit, over the buffer capacitor means that the DC supply voltage is clamped to. Since the fraction is decreased, the DC voltage over the buffer capacitor means will have to increase to a higher value (the second predetermined value) before the DC supply voltage reaches the predetermined lock out voltage and the control circuit is reactivated. It has been found that the fraction decrease circuit can be realized in a relatively simple way, in case use is made of a transistor.
Since TRIAC dimmers are normally used in combination with incandescent lamps, the ballast circuit according to the present invention is particularly suitable to be used in a compact fluorescent lamp, since compact fluorescent lamps are often used to replace incandescent lamps and are equipped with the same lamp sockets as incandescent lamps.